LT8331 datasheet(10/30 Pages) AD

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LT8365

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

Figure 1. Burst Frequency vs Load Current

ACHIEVING ULTRALOW QUIESCENT CURRENT

To enhance efficiency at light loads the LT8365 uses

a low ripple Burst Mode architecture. This keeps the

output capacitor charged to the desired output voltage

while minimizing the input quiescent current and output

ripple.InBurstModeoperation,theLT8365deliverssingle

small pulses of current to the output capacitor followed

by sleep periods where the output power is supplied by

the output capacitor. While in sleep mode, the LT8365

consumes only 9µA.

As the output load decreases, the frequency of single cur-

rent pulses decreases (see Figure 1) and the percentage of

time the LT8365 is in sleep mode increases, resulting in

much higher light load efficiency than for typical convert-

ers. To optimize the quiescent current performance at light

loads, the current in the feedback resistor divider must be

minimized as it appears to the output as load current. In ad-

dition, all possible leakage currents from the output should

also be minimized as they all add to the equivalent output

load. The largest contributor to leakage current can be due

to the reverse biased leakage of the Schottky diode (see

Diode Selection in the Applications Information section).

While in Burst Mode operation, the current limit of

the switch is approximately 325mA resulting in the

output voltage ripple shown in Figure 2. Increasing

the output capacitance will decrease the output ripple

proportionally. As the output load ramps upward from

zero the switching frequency will increase but only up

to the fixed frequency defined by the resistor at the

Figure 2. Burst Mode Operation

RT pin as shown in Figure 1. The output load at which

the LT8365 reaches the fixed frequency varies based

on input voltage, output voltage, and inductor choice.

PROGRAMMING INPUT TURN-ON AND TURN-OFF

THRESHOLDS WITH EN/UVLO PIN

The EN/UVLO pin voltage controls whether the LT8365 is

enabled or is in a shutdown state. A 1.6V reference and a

comparatorA6withbuilt-inhysteresis(typical80mV)allow

the user to accurately program the system input voltage

at which the IC turns on and off (see the Block Diagram).

The typical input falling and rising threshold voltages can

be calculated by the following equations:

VIN(FALLING,UVLO(–)) = 1.60 •

R3 + R4

VIN(RISING, UVLO(+)) = 1.68 •

R3 + R4

VIN current is reduced below 1µA when the EN/UVLO pin

voltage is less than 0.2V. The EN/UVLO pin can be con-

nected directly to the input supply VIN for always-enabled

operation. A logic input can also control the EN/UVLO pin.

When operating in Burst Mode operation for light load

currents, the current through the R3 and R4 network can

easily be greater than the supply current consumed by the

LT8365. Therefore, R3 and R4 should be large enough to

minimize their effect on efficiency at light loads.

VIN = 50V, VOUT = 100V, BOOST

LOAD CURRENT (mA)

200

400

600

8365 F01

2µs/DIV

VOUT

20mA/DIV

IL1+ IL2

100mA/DIV

8365 F02

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